I couldn't help myself... it looked so lonely there on EbayIt's true that I don't have a place for it in my system but it can be a backup in case one of my other amps craps out.

All of you enablers here aren't helping my situation...

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Hey y'all, One of my other amps (not the one I just bought) was living on borrowed time (could crap out any day): I opened it up to inspect the caps and found burned areas all over the PCB board (see TMF-25-2 Service Bulletin). The moral of this story is that if you see another amp for sale, BUY IT!

It's true. I just bought an M-500t. There was absolutely nothing wrong with it, but a look see on the inside showed a little electrolyte leakage around the main power supply caps and bulging tops. Fortunately I had planned on recapping anyway, because it had never been serviced (well, maybe once, because the emitter resistors had been changed, but that could have been a production run change, but the caps were original). Just got my parts today from Digi-Key.

Update: Since my last post, I have replaced all of the electrolytic caps in my three TFM-25's and my C1 preamp. I am very pleased with the improved dynamics and the sweet sound. I have a lot of confidence in my equipment now and I can't tell you how empowering this project was. Special thanks to BillD and RichP for helping me out. I would post a write-up and pics but I can't figure out how to upload the pics here.

I had already done a full writeup with multiple pics but wasn't able to just copy & paste it here in-mass and I didn't want to redo it piecemeal. I suppose I could just post a link to my writeup on "that other site" but I figured that might not go over too well here. I'll get around to reposting it here it when I have the time.

I've always been intrigued by electrostatic speakers but never could afford them. I was inspired by some guys on the DIY Audio Forum rolling their own electrostats and they convinced me to build a pair for myself. I bought Roger Sanders' book, "Electrostatic Loudspeaker Design Cookbook", and dived in.

After a great deal of study and deliberation, I devised a hybrid design using a 10” woofer in a transmission line (TL) alignment with a 1ft x 4ft flat-panel electrostat positioned above the woofer.

My initial concepts for a compact TL small enough to merge with the stat panel in a unitary enclosure/frame did poorly in modeling by GM at the DIY Forum and my choices narrowed to accepting a smaller but less capable sealed or ported box or a larger TL box. The challenge for the TL option was cramming all that volume into a reasonable footprint while also placing the woofers at best location to blend with the stats. My solution was extending the box volume upward, behind the stat panel. Of course, a flat box surface there would bounce the stats’ backwave back to the stat and degrade its sound—so, I formed the box (behind the stat) into a “V” shaped “beam-splitter” which diverts the backwave out the open sides of the speaker.

I chose the 10" Aurum Cantus MkII woofer for it’s nice construction and wide frequency response (20hz-1khz), as the woofer had to produce both bass and lower midrange in this design. I opted to cross the woofer over to the stat panel around 380 hz to mitigate the low frequency phase-cancellation or “suckout” which is characteristic of dipole electrostat panels The woofer is canted upward 6 degrees and mounted at the front of the 4ft3, 9ft long folded and tapered transmission line. The bass would be uncompromised in this design.

The heart of each stat panel is a whisper-thin (6-micron) Mylar diaphragm which is sandwiched between 18-gauge perforated steel stators and assembled with 3-M 1/16” UHB foam mounting tape. The stators are spray-coated with 12 mils of polyurethane paint for insulation. The diaphragm is coated with a proprietary high-resistance conductive coating to carry a 2.5KV DC bias charge supplied by a dedicated power module that plugs into a 120VAC wall outlet. The stators receive their high voltage AC music signals through an EI-core step-up transformer with 100:1 windings. All of these specialty items were purchased from Russ at Just Real Music. Overall speaker dimensions are 15" wide x 67 1/2" high x 21 1/2"depth.

The speakers are vertically bi-amped using a DBX model 223 active crossover with 24db slopes set for 380hz, feeding a pair of Carver TFM 25 power amps. In lieu of a dedicated shelving circuit, the stats’ dipole phase cancellation is compensated using my system’s Audio Control C101 EQ.

The Moment of Truth:I fired these babies up on July 4, 2008 with [my Goddess] Diana Krall singing Nat King Cole's "You're looking at me". The illusion of Diana performing live in my living room was so scary-real I could almost smell her perfume during that first tune. Finally I have speakers worthy of Diana, who now sounds as good as she looks

The TL bass is full and tight and fast with less coloration than any speaker I’ve owned and blends surprisingly well with the electrostats. In the second tune, Diana singing “Peel Me a Grape”, the bassist does this wonderful riff with a first long note fretted upscale followed by two quick notes stepping down and down again and the Aurum Cantus woofers were right there all the way-- it was just yummy and affirmed my choice for the transmission line bass alignment.

The 4ft2 flat-panel stats are incredibly fast and detailed. Even at the lowest volume levels every nuance is heard. They also play to ear-splitting volumes with no loss of accuracy or hint of distress. They are, however, ultra-directional with a very narrow sweet spot (one person wide)—so, not a good choice for party speakers.

In their sweet spot, however, the 3D imaging is quite magical with exquisite detail and downright stunning speed. A friend described them as “remoteheadphones”. A woman’s voice thru these stats takes my breath away and Diana's puts me on my knees.

Ironically, these speakers’ greatest asset is also their greatest flaw: I’ve concluded that flat-panel stats have such phenomenal imaging precisely because they beam sound like a laser; limiting late/reflected sounds reaching the listener. So, whether their beaming is a fault or a virtue depends on your perspective.

A pink-noise frequency response sweep with my Audio Control EQ in RTA mode revealed a roll off above 5 khz and I had to add 8db’s of boost in the 10k-20k band to correct it. I’ve since learned that the higher transformer step up ratios are needed for full range stats that have to play below 200 hz while the lower step-up ratios provide adequate bass output in hybrid applications while providing superior treble response. In retrospect, a 50:1 or 75:1 transformer would have been a better choice for my hybrid. At this time I am considering changing to tandem pairs of 50VA 230V/6V European power toroids wired backwards to give a 68:1 step-up ratio.

Interestingly, the sound emanating from the stats doesn’t seem to decrease with distance—it sounds about the same from 1 foot as it does from 20 feet. However, the balance between the woofer and stat outputs does change a bit with distance so precise balance only exists at the sweet spot. Moving out of the sweetspot in the horizontal plane, the highs roll off progressively and dramatically because the stats beam treble like a laser. Moving to another room, the sound decreases as expected but seems perfectly balanced and carries quite well.

Conclusion:Inside their sweet spot, no conventional speakers I’ve heard can match these for realistic imaging, detail and speed. Moving outside of their sweet spot, they are still clean and listenable but progressively unbalanced and less impressive.

I live alone so there's no competition for the best listening position. With the speakers precisely aimed and equidistant from my usual spot on the sofa, I’m always in the sweetspot, so their shortcomings outside of that zone are not an issue for me—and I do LOVE their sound.

If you can accept their narrow sweet spot, flat-panel electrostats provide a scary-real, almost magical listening experience.

Mylar diaphragm on MDF/bike tube tensioning jig. Mylar diaphragm was wrapped over jig and secured on backside with tape. Tensioning was accomplished by inflating the bike tube with a hand-pump. I used a fine tip marker pen to place reference marks 6" apart on the diaphragm, then applied tensioning until the distance between marks measured 6.060 to achieve 1.5% elongation. Holding my breath here because the Mylar was stretched very close to its breaking point.

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Stator with tensioned diaphragm installed: 1/16 thick 3M foam mounting tape secures the diaphragm to the stator. At this time a proprietary high/resistance conductive coating was applied to the diaphragm to carry the 2.5kv bias voltage. The coating solution was obtained from Russ at Just Real music (available online).

Rabbit, Engtaz, Thanks for the complements guys. As I've said before, it still amazes me that you/me/anyone can take a couple of perforated metal plates and a sheet of plastic and some tape and build a sound transducer on your kitchen table that will rival any high-end commercial product out there. Electrostats lend themselves well to DIY and the materials are not all that expensive so I'm surprised more people aren't building them. I invested maybe $750 in mine-- without the elaborate cabinets they could be built for a lot less. Most DIY'ers use either a separate TL cabinet or a small ported woofer cabinet under the stat panel like Martin Logan uses. I've not seen any that use a beam splitter TL like mine. Really, the only hard items to get at a reasonable price are the step-up transformers and you can get a pair of 50:1's from Just Real music for $58 each. The perforated steel for the stators cost me $110 and I later found some 40" perf steel for even less at McMaster Carr. The only significant ancillary expense is that you would really need to use an active crossover and bi-amp them because electrostats present a capacitive load with frequency-dependent impedance so it's practically impossible to design a passive crossover that works well with them. Even so, if you already have two amps, a good adjustable active crossover can be had for around $75 on Ebay. I've attached a couple of images which show the internal configuration of the transmission line and beam splitter.

weitrhino wrote:Very impressive work, jazzman. But how did you decide on the length and volume of the transmission line for the bass port? That kind of math screws with my head.

Thanks Weitrhino. That kind of math screws with my head too. The definitive expert is Martin J. King and I had his formulas but couldn't figure out how to apply them. Thankfully a really smart guy at the DIY Audio Forum shot down my earlier compact line concepts so I didn't screw it up. Eventually I gave up on the math, went with a larger box and just used the generic guidelines for transmission lines included in Roger Sanders' "Electrostatic Loudspeaker Design Cookbook". Basically, Sanders's guidelines are:1) Make the box walls behind the woofer curved or angled-- never parallel to the baffle board. 2) Make the line at least 8ft long. If absolutely necessary, a shorter line can be used by overstuffing with fiberfill to slow the speed of sound thru the line. 3) Line sectional area at woofer end should be < 125% of woofer's driven area.4) Line sectional area at port end should be 75% of area used at woofer end. 5) A straight taper from front end to port end is preferable. (volume is then set by the front & back sectional areas and the taper between them)6) Stuff the line with wool or polyfil to .5 lb/ft then add or subtract as needed for tuning.

The REAL advantage of a TL is not so much the lowered bass response but the tonal character resulting from the almost total absence of resonance in the box as compared to other box designs. After I built these speakers I built a sealed-box subwoofer and I posted a write up of that project on another forum. To explain my thoughts on box resonance and their effects, here is an excerpt from my subwoofer write-up:

"This is gonna sound funny but whenever I finish stuffing a speaker box with polyfil, I always stick my head in the woofer opening and start humming up and down the octave scale while listening for any resonances (not that there's anything I could do about it-- I'm just curious). In this box, the resonant pitch rebounding to my ears rattled my skull big-time. At that very moment I had an epiphany about the huge resonance difference between this box and the open-ended transmission line boxes used in my hybrid electrostats— I thought about how the line curvature behind the woofer in the hybrids provides no parallel surfaces to bounce sound back to the woofer and how the stuffed line vents the backwave to eliminate compression effects. Of course, I knew all that at an intellectual level when I chose a transmission line for those speakers but actually hearing the difference rattling my skull in this sub box drove it home to me. When I hummed into one of those transmission lines I got no sound back at all-- it was graveyard-dead. The woofers in those hybrids also produce the low end of the midrange and it's now so clear to me how they can produce those frequencies so beautifully. "